1. Field of the Invention
This invention is directed to hydrophilic terpolymers and to shaped hydrogel articles, especially contact lenses, produced therefrom.
2. Description of the Prior Art
Interest has arisen in recent years in hydrophilic plastic materials for use in the manufacture of contact lenses because in the hydrated state the polymer is soft and jelly-like and could be expected to be tolerated more readily than conventional hard plastic materials such as methacrylates. Thus, Wichterle et al. in U.S. Reissue Pat. No. 27,401 discloses a hydrogel, allegedly containing up to 97% by weight of water, which is formed from a hydroxyalkyl acrylate or methacrylate sparingly crosslinked with the corresponding diester. The amount of water which such hydrophilic polymers are capable of retaining is, as stated by Wichterle, inversely proportional to the degree of crosslinking so that the hydrogels having a high water content can only be obtained when the extent of crosslinking is very small. Other compositions including polyvinylpyrrolidone (PVP) graft copolymers have been developed for contact lenses but the significant solubility of the PVP structure in water over a relatively short period of time has created undesirable characteristics in such lenses. U.S. Pat. No. 3,532,679 to Steckler discloses a crosslinked copolymer comprising a vinyl lactam and an acrylic ester monomer, but the amount of lactam required in the composition is so high as to affect the mechanical properties of the resulting lens product, which is described as containing up to 95% water. High water content contact lenses such as proposed by Wichterle and Steckler are characterized by relatively high rates of transmission of gaseous metabolites to and from the cornea but this positive feature is countered by their poor mechanical properties. Hydrophilic contact lenses having less than 50% water content have good mechanical properties but do not exhibit sufficiently high oxygen transmission characteristics to facilitate permanent wear. Thus, in order to prevent anoxic edema such lenses must be removed during non-waking hours.
Therefore, the need exists for contact lenses possessing good corneal respiration along with good mechanical properties.